1. Technical Field
The present invention relates to an antireflection film which is formed on one face of an optical component such as a lens or a filter to produce an antireflective effect on light of a predetermined wavelength band.
2. Description of the Related Art
In an imaging apparatus such as a photographic camera or a broadcast camera, usually, many optical components such as lenses, prisms, and filters are arranged along the optical path. When light is incident on the surface of such an optical component, part of the light is formed as reflected light. When the total number of the optical components is large, the total amount of reflected light is correspondingly increased, and hence problems such as flare or ghost are caused in, for example, a broadcast camera. The reflectivities at the surfaces of the optical components are distributed with respect to the wavelength of the incident light, and show various wavelength dependences according to the constituent materials of the optical components. Therefore, the chromaticity balance is impaired, and the white balance must be adjusted in the whole imaging apparatus.
To comply with this, conventionally, an antireflection film is disposed on the surface of each optical component.
An antireflection film is a multilayer film formed by a combination of dielectric films having different refractive indices. For example, antireflection films are disclosed in JP 2002-107506 A, JP 2001-100002 A and JP 2002-267801 A.
In the antireflection films disclosed in JP 2002-107506 A, JP 2001-100002 A and JP 2002-267801 A, however, the reflectivities in the vicinities of the upper and lower limit (i.e., the vicinities of 400 nm and 700 nm) of the wavelength band which is usually deemed as the visible light region tend to be slightly impaired. For example, a wavelength band in which the reflectivity is 0.15% or less is limited to a width of about 280 nm. Therefore, an antireflection film which can exhibit an excellent light transmittance in a broader wavelength band is requested.